Automotive vehicle braking system



Dec. 25, 1951 SAHLGAARD 2,579,616

AUTOMOTIVE VEHICLE BRAKING SYSTEM Filed Dec. 20, 1946 4 Sheets-Sheet l Dec. 25, 1951 A. D. SAHLGAARD AUTOMOTIVE VEHICLE BRAKING SYSTEM 4 Sheets-Sheet 2 Filed Dec. 20, 1946 INVENTOR. BY :Yd/r czarci G my Dec 25, 1951 1 SAHLGAARD 2,579,616

AUTOMOTIVE VEHICLE BRAKING SYSTEM Filed Dec. 20, 1946 4 Sheets-Sheet :5

Dec. 25, 1951 SAHLGAARD 2,579,616

AUTOMOTIVE VEHICLE BRAKING SYSTEM Filed Dec. 20, 1946 4 Sheets-Sheet 4 INVENTOR.

Patented Dec. 25, 1951 2,579,616 6 AUTOMOTIVE VEHICLE BRAKmGsYs'rEM Aage D. Sahlgaard, Chicago, Ill.

Application December 20,1946, Serial No. 717,530 1 Claim. (01. 188- 3) This invention pertains to new and useful im.-

provements in braking systems for tractortrailer combinations, buses, trucks, and similar heavy vehicles. chanical means cooperative with the conventional braking system for locking the brakes of equipment of the above described type under predetermined conditions. Its operation is designed to be either automatic'or operator con trolled. The invention is adaptable to be an integral part of the original brakingsystem or may be addedto such system as an accessory.

For better understanding of the conditions under which this invention finds use, reference first will be had to a typical tractor-trailer unit. Such a unit usually comprises a four wheel tractor, having a pair of front or steering wheels and at. least one pair, of rear driving wheels. The front end of the trailer body usually reposes on a so-called fifth whee1on which the trailer may have pivoted, tractor-following, horizontal movement. The rear end of the trailer is borne by normally freely revolving rear wheels.

A tractor-trailer unit has considerable weight in and of itself and under normal operating conditions much additional poundage is carried in thetrailer. Since suflicientbraking force could scarcely be built up in the tractor alone to safely and, effectively controlthe entire vehicle, trailers now all instances have been equipped with brakes. hich are controlled by the driver, for their operation making use of generally the same service, brake operating means as those serving topperate the tractor brakes.

Although other systems such as electric and hydraulic are occasionally seen, the most commonly used types of braking systems for large,

heavy vehicles, Whether of the tractor-trailer type 0l conventional ,truck or bus type, are the so-called air brakes or vacuum brakes}? Air brakesrely for their actuating force upon compressed air, the compression of which is built up, by well known types of compressors in driven association with the vehicleengine, the compressed air being stored in one or more suitable pressure tanks.

Vacuum brakes rely for their activating force upon the partial vacuum generated under normal operating conditionsbythe suction built up in the intake manifold of thevehicle engine. To maintain a continuous partially vacuous condition always ready for operation of the brakes, a vacuum reserve tank is utilized, the air in which is normally maintained in partially exhausted condition below atmospheric pressure.

. In either of the above mentioned type of braking: systems the footbrake pedal usually operates avalve, closing it in undepressed condition and opening, it in depressed condition, the valve being so] arranged in the air pressure or vacuum lines as to makefaccessible theair .pressureor vacuum to, suitable cylinders or chambers in which are In particular, it pertains tome- 2 V positioned suitable pistons, diaphragms, or the like, the movement. of which, under the force of compressed air or air pressure in the presence of a partial but sufficient vacuum, serves to operate the brake activating levers and associated mechanism. Usually as an additional safety precaution a hand operated emergency brake is provided for the tractor of a tractor-trailer combination, or for the entire brakin system of a truck or bus. In some instances the emergency brake is mechanically articulated with the brake activating rods, tending to set the brakes when pulled, and operating generally through the same brake activating linkage as is operated by the piston under the pressure of compressed air;or that of normal atmosphere in the presence of' 1 the built-up vacuum.

More generally, however, the emergency brakes of tractors, trucks and buses are operative only on the drive shaft of the vehicle. As indicated, the trailer of tractor-trailer units are not usually provided with mechanically operable emergency brakes but are entirely reliant on the compressed air or vacuum brake system, whichever they may have, for their brake operation. This is due to the articulated relation of the trailer and tractor which makes brake-operating mechanical connection between them very difficult" to achieve.

No difficulty is encountered in providing a flexible compressed air or vacuum line hose between tractor and trailer, in communication between the tractor braking system lines and the 1 This is the com-1 mon practice, that is to say, that the trailer is operating cylinders trailer braking system lines.

usually providedwith brake or the like which, in road operation, are put into communication with the regular tractor brake:

system by being coupled thereto, the trailer brakes then being cooperatively and simultaneously operable with those of the tractor through the tractor braking system operating members.

As an additional safety factor for the trailer: particularly in the event it should accidentally,

become detached from the tractor during operation, an emergency braking arrangement is usually provided, which generally includes areserve compressed air or vacuum tank, and relay valve means associated therewith which keeps thisfbrake operating force normally inoperative during. effective operation of the tractor braking;

means, but causes the reserve of compressed air or vacuum to become immediately operative to set the brakes when the regular brake operating communication to the tractor is tally or purposely broken.

When the trailer is detached from the tractor, whether purposely oraccidentally, the brakes are automatically set and held in locked condi-'- tion under the operative forces placed upon suit.- able brake operating mechanism by the reserve air pressure or yacuum in the trailer compressed either acciden 3 air or vacuum tank. The brakes will remain locked as long as there is sufficientair pressure or vacuum in the system to maintain them in that condition. The length of time the brakes will remain locked is a variable depending much on the condition of the trailer air or vacuum lines and the condition of the brake operating I cylinders. V Commerce Commission has laid down rules that the brake operating system must be'capable of. locking the brakes of a trailer disengaged from particularly when (as often happens) a trailer must be left disengaged from its tractor for a considerable period of time. As the compressed air or vacuum is diminished due to normal leakage, this being the only means of holding the trailer brakes locked, the results can often be serious, particularly if the trailer is parked on a grade.

Referring now to the brake systems of nontrailer type heavy vehicles such as trucks, buses and. the like, it is quite conventional for the air o'r" vacuum brakes to be operated by the foot pedal and for the emergency brake to be articulated with the normally air or vacuum operated braking rods by mechanical means. In other instances, the emergency brake may have operative mechanical connection only with a brake operative on' the transmission or the drive shaft. In many'instances the, emergency brake operating mechanism is inadequate for the stresses put upon it particularly at times when the heavy vehicle must be stopped and permitted to remain at a standstill on a severe grade, or, as another example, under conditions such as motor failure, resulting in loss of air pressure or vacuum, or other circumstance wherein constant application of the service brakes is impossible or impractical. I a

"In view of the foregoing, the objects of the present invention are to overcome the deficiencies of the prior art by providing a device capable'of installation as an accessory or as an integral part of newly manufactured equipment which, in the case of trailers of tractor-trailer equipment, will serve to automatically and continuously lock'the' brakes of such trailer when thetrailer is detached from the tractor; and which, if desired, will act as a manually controllable brake lock, operable on the tractor alone when the trailer. is disconnected therefrom or upon all the brakes of the tractor-trailer combination when the two units are connected together/supplementing. the conventional tractor emergency brake equipment.

How the foregoing and other objects and advantages, apparent to those skilled in the ,art, are achieved is set forth in the detailed description which follows and on the drawings in which:

Fig. 1 is a schematic showing of the invention Suffice to say that the Interstate brake drum, brake shoes and shoe activating cam, shown for. better understanding of the operation of the device. comprising the invention;

Fig. 4 is a side elevation, partially in section, of a conventional adjustable brake rod lever or slack adjuster which, like the showing of Fig. 3, is incorporated for greater clarity in the disclosure which follows;

Fig. 5 is a diagrammatic side elevation, partially broken and partially in section, showing a typical form of brake locking means used in conjunction with a compressed air type braking system;

Fig. 6 is a diagrammatic side elevation, partially broken and-partially in section, showing a typical form of brakelocking means used in conjunction with a vacuum type braking system; and

Fig. '7 is a diagrammatic showing, partially in section, of a modified form of the brake actuating shaft and dog cooperative therewith, a different form of the first mentioned of which is shown in Figs. 4 and 5.

Referring now to Fig. 1, reference numeral I indicates a source of either compressed air or vacuum, in the former case being a compressor inoperative engagement with the vehicle motor' and in the latter case the source of vacuum'being the engine intake manifold. Element l is in communication through suitable piping with an air or vacuum reservoir 2 and suitable valve. means (not shown) operates to' retain in the lat-. ter a relatively constant supply of compressed.

air or vacuum.

A typical conventional brake operating mechanism will next be described for better understanding of the overall operation of the inven:

tion. The brake system about to be described is one commonly found on tractors, buses and.

chambers 6 each of which has piston-like means within it which is operative under compressed or atmospheric air pressure to move in proper direction the outwardly extending brake rod 'l, one

end of which is attached to the piston-like means i for reciprocation therewith.

At its end opposite its connection to the pistonlike'means each brake rod 1 is connected we slack adjuster 8 (Fig. 4) ,which has one or more holes 9 adjacent its outer end to receive the end of said brake rod. 7 As shown in the broken lower portion of Fig. 4, each slack adjuster'is' bored out to receive a spiral gear I0, the: inner and outer ends of which are cut down to form opposed axles ll, l2, respectively. Axle l2-extends outwardly of a screw cap l3 which is centrally bored to receive it. The outer end of axle. l2 extends outwardly of screw cap l3 and is so shaped at Mas to be capable of being turned with a wrench for adjustment of' its position with re spect to the hereinafter described parts which cooperate with it. 'Spiral gear It is meshed with theteeth of an other gear l5 and the latter is rigidly attached adjac'ent the outer end of a brake cam operating shaft I6; as particularly shown in Fig. 2. As

shown in Figs. 2 and 3, at its end opposite to slack adjuster 8, shaft l6 carries a cam l1 which is medially positioned between the opposed free ends I8, I8 of a pair of i generally semi-cylindrical brake shoes 29, 29 which have brake lining 2|, 2| attached thereto along their outer peripheries. Each brake shoe is pivoted as at 22 to the brake drum 23 and a spiral tension spring l9 extends between the shoes 20, 20 to normally hold them out of contact with the out-turned annular inner braking side or rim 24 of brake drum .23. Brake drums 23 are attached to the vehicle wheels in conventional manner as shown in Fig. 2. I

When foot pedal 4 is depressed, air or vacuum is released from tank 2 through the air or vacuum lines 2 and 3 shown in Fig. 1 to all the brake operating cylinders 6, and movement is thus imparted the piston-like means within such cylinders, imparting movement through brake rods 1 and slack adjusters '8 to brake cam operating shaft 16. Movement of each shaft 6 is imparted to cam |1, spreading brake shoes 29, 29 apart and putting brake linings 2|, 2| into interior surface-contacting, frictional engagement with the annular rim 24 of brake drum 23, thereby applying a wheel-retarding braking force to each wheel.

Cam operating shaft M6 is normally supported for partial rotation by a bracket (not shown) which is mounted on the vehicle axle at a point between the brake drum 23 and the slack adjuster B. In utilizing my invention I substitute for the conventional cam operating shaft support bracket the cam shaft or brake-locking means 25 shown in Figs. 2, and 6 and hereinafter more fully described.

As hereinbefore mentioned, my device may be added as an accessory or may be conventional equipment. In the former case, the device 25 is preferably mounted on the axle by an axle encircling yoke 26 attached to the device by a neck portion 21, and in the latter case may be attached to a mounting integral with the conventional vehicle axle (not shown). The manner of mounting is relatively unimportant except that the cam operating shaft locking means must be rigidly attached to the vehicle axle to properly support the shaft l6 and otherwise properly operate in the manner hereinafter described.

Referring now to Fig. 5, it will be seen that the cam operating shaft locking means therein shown comprises a lower outer housing 28 and an outer upper housing 29, the former being integral with neck 21. Lower outer housing 28 is dished out, as shown, to contain an inner lower housing 3| having a threaded air inlet 30 formed therein which extends outwardly through an opening in said housing 28 for connection to any one compressed air line shown in Fig. 1 as leading thereto. Inner lower housing 3| conforms generally to the shape of outer lower housing 28. Extending across the upper out-turned margins of lower housing 28 is an air impervious flexible diaphragm 32, the peripheral edges of which are impinged in air tight manner by the outwardly extending opposed annular lips'33, 34 of lower and upper. outer housings 28, 29, respectively. The annular lips 33, 34 are held in air tight abut- ,It will be obvious that, should it be desired, upper inner housing 31 and lower inner housing 3| could be dispensed with by redesigning upper housing 29 at its neck portion to have an integral annular shoulder similar toshoulder 38 to form an abutment for the upper end of spring 40.

When an air line 11 is attached to connection 39 there is formed between inner lower housing 3| and diaphragm 32 an air chamber which, under proper air pressure through the air line, causes diaphragm 32 to assume an. upwardly extending generally horizontal expanded position such as shown in full lines in Fig. 5. In the absence of sufficient air pressure diaphragm 32 tends to collapse in the manner generally indicated in Fig. 5 by the dotted lines. Still referring to Fig. 5, above diaphragm 32 is an upper inner housing 31 having formed thereon at its upper portion an annular shoulder 38 in which is formed a central opening 39. One end of a helical compression spring 40 abuts against the lower annular .face of shoulder 38 and the other end, of such spring abuts on the upper annular face of ,a head 42 of a vertically extending lever 4|. The lower face of head 42 engages the upper face of diaphragm 32, for movement therewith under the urging of spring 49. The upper end of lever 4| extends through opening 39 in shoulder, 38 and is connected tothe end 42 of a dog or latch 44 by means of a stub shaft 43. A cotter key or similar fastening means prevents disengagement of stub shaft 43 from lever 4|. 8 Dog 44 is medially pivoted on a stub shaft 45 to a bracket 46 preferably formed integral with upper outer housing 29 and a cotter key or other fastening means prevents disengagement of stub shaft 45 from dog 44 and bracket 46. At the free end of dog 44 is a book 41, as shown, for engage: ment with the outwardly protruding peripheral teeth 48 of a collar 49 which is rigidly attached to cam operating shaft |6. Hook 41 is so shaped as to permit rotation of shaft It in the brakesetting direction shown by the arrow in Fig. 5 when engaged .withteeth 48, but prevents rotation of such shaft in the opposite or brakere leasing direction unless disengaged in the manner hereinafter described. A covering plate 50, aflixed to upper outer housing 29 by machine screws 5|, permits access into said housing for repair or adjustment of the parts contained therein should it be required. i

It will be obvious from theshowing of Fig. 5 and the above description referring thereto that upon collapse of diaphragm 32 under lack of air pressure, lever 4| will be forced downwardly by the compression spring 40 and such movement will be transmitted through the linkage shown to dog 44, causing hook 41 to engage teeth 49. As previously set out, shaft |6 may still rotate in the one brake-setting direction but cannot move back into brake-releasing position until diaphragm 32 has been returned to generally horizontal position under air pressure.

Referring now to Fig. 6, the modified form of cam operating shaft locking means therein shown is a vacuum operated mechanism comprising a lower outer housing 52 in which is a lower inner housing 53 having a vacuum connection 54 which extends outwardly through an opening in housing 52 for connection to a vacuum line. A flexible diaphragm 56, held normally in depressed position by atmospheric air pressure on the side opposite that to which vacuum is applied, extends across the upper annular periphery 51 of lower outer housing 52. The lower annular peripheral edges 58 of the upper outer housing 6| Behaving an annular'shoulder 64 formed at the upper part thereof. Medially of shoulder 64 is an opening '65 through which passes a lever so having'a lower head portion 61 formed thereon. Head portion "51 is affixedto diaphragm 5B and a helical tension spring 68 is 'aifixe'dat its one end to the upper surfaces of head portion 61 and at its other to the lower surfaces of shoulder 6'4 'to'constantl-y urge diaphragm 5'6 and lever 66 to resist the vacuum-withdrawn, normal conditionof such diaphragm and to assume that position shownin-dotted-lines with respect to such parts.

Ati-ts outer end lever 66 is attached to the me' dial portion of 'a dog 'or latch 69 by a stub shaft 10. One endoi dog B9 is fixed for limited vertical movement to a bracket H formed integral with upper-outer liousing 6| on stub'sha'ft 1-2. To prevent displacement fromthe parts to which they are attached, stub shafts 10, 12 have cotter-keys or other retaining means associated therewith. the outer end or dog 69 is a hook 13 which, as 'shown by dotted lines in Fig. 6, is adapted to engage the peripheral teeth 48 'of the collar 4'9, the latter being rigidly attached to brake cam operating shaft I6 in the same manner as that described with respect to the showing of Fig. 5, i. -e., brake-setting rotation of shaft 16 is permitted but brake releasing rotation is eifectively prevented when hook 13 is in engagement with A--covering plate 74 affixed to upper outer housing 8| by machine screws Sip'er'mits access into-said housing for repair or adjustment of the "parts contained therein should it be require'dr' From -the above description that when a sufficient supply of vacuum IS available in the chamber formed -between lower inner housing 53 and diaphragm 58 such diaphragm will continue to assume that expanded condition shown in full lines in Fig. 6. How- -ever,"the instant the vacuum is for any reason withdrawn spring '68 will withdraw coincident with collapse of diaphragm 56, bringing with it lever 66. This action will throwhook 13 into br'ake lockin'g engagement with teeth '48, locking the brakes in set 'conditionin the manner hereinafter described, until vacuum is again restored to the unit. 7

Referring now to Fig. 7,"there is shown therein' a modification of the separate collar-withperipheral-teeth arrangement (elements 49, &3,

ernce numeral '69 in Fig. "6. The arrangement shown in Fig. 7 is particularly adapted for in instances wherein the invention is initially appliedas an inte ral original part of a vehicle brake system.

Referring again to Fig. l,. the schematic'shoim ing therein may be taken as being applicable it will be apparent to eitheria self=coiitained unit 'suchasa bus or thick, or as applicable toa tractor alone or as applicable to a combined connectedtractor-trail 'er unit. '-By reference to the leftmost portion'of this figurewholly eliminating from consideration ai'r lines 18, it and the remaining structure at the right of said figure there, is illustrated a typical installation of the inventionas applied to a truck or bus; :by including airlines 18, 19 and shut-off valves -80, -80, application ;of the invention to a tractor disconnected from its trailer may be envisioned; and thisifigure taken in its entirety illustrates a connected-together tractor and trailer.

To the extend described in this paragraph the vehicle may be considered as being a truck or bus. Itwill be noted that a manually operable valve {H is interposed in the air or vacuum-lines 33 leading from air or vacuum storage tank 2 to each of the tour brake actuating-shaft lock-1 ing means 25 shown as being associated with the tractor or busbraking system. Valve His normally open thereby making accessible tothe four brake-locking units 25 of the vehicle the full amount of compressed air or vacuum in tank '2; Upon being closed, valve it shuts oh? the supply ot-eompressed air or vacuum and simultaneously opens up an exhaust port formed therein not shown) to permit exhaust of -air or vacuum from the lines therebeyond leading to brake-locking devices jZB When valve 7'! is closed the diaphragms 32 (01' '55) collapse and the hooks such as those shown in Figs. 5, 6 or 7 are permittedto-engage the teeth (48 or '15) asso ciated with the brake cam operatingshafts Hi. This, of course, will not of itself set the brakes. but they may be then set in conventional manner by depressing foot pedal 4. However, irrespective of whether the service brakes are set by the conventional means previous to, simultaneously with or subsequent to closure of valve 11 (since one-way, brake locking movement of shaft is may be had even 'in-the engaged position of the hooks and teeth) the bra'kes will then be locked in setposition and will remain "so until disengaged in the manner hereinafter "set out. The foregoing description of the manner of locking the brakes of a truck or bus is equally applicable to a tractor from which the trailer is purposely detached subsequent to closure of shut-off cocks o'rvawes 80, 8 0, as hereinafter described.

st'ii'l referring'to Fig. "1, it win be 'no'ted "that a s'o-ca11eeserviee air .or 'vacuum -line 18,18 leads from foot pedal operated valve 3 toan emergenc relay Valve 8| on the trailer. suitable conventional emergency relay valve, herein diagrammatically illustrated and des ign'a'te'd by numeral 8|, may be employed, "as

for-exam le, the relay valv shown in Figs. 6 and 7 of Patent No. 25151.13}? or the relay jvalve shown 'in Figs. 2 and 3 of Patent NQJQBSKLSOW. so-called emergency air o'r vacuum line 1'9, 19' leads from normally open communication with the air or vacuum tank side of the line to- "said relay valve 8|. When the emergency line is'directly connected to air or vacuum tank '2 by *a Iine l'Q, T9 in the manner shown on the V drawings, closure of valve l will be effective for brake iocking purposes only upon the tractor bralces iirthe manner described above. new-av i when "the'trailer emergency line c nnection is madeto thelin'jes beyond valve M, as indicated by-the dotted line connection designated '19"; ensure of-Valve Tl Will beefiecuve Cause "01)- eration of all the brake operating shaft locking means associated with both tractor and trailer, permitting all the brakes of both units to be set and locked simultaneously.

Under the conditions outlined above, brake locking means 25 will then effectively hold the brakes in set position. Reopening of valve ll in the presence of a built-up supply of air pressure will reexpand the diaphragms of units 25, causing the parts associated therewith to assume inoperative, non-locking position. Deactivation or release of the brake locking units is sometimes facilitated, after build-up of air or vacuum pressure in the lines, by reapplication of the conventional brake-setting means, this tend ing to loosen up the entire brake setting and locking mechanism.

Both the trailer service brake and emergency brake lines are each provided with a shut-on" cock or valve 8t which valves are normally open when a trailer is attached to the tractor and which are manually closed when the trailer is detached in the usual course of handling.

As mentioned hereinabove, both the service air or vacuum line l8, l8 and the emergency air or vacuum line l9, 19' are in communication with an emergency relay valve 8|. Emergency relay valve 8! serves to cause the build-up, during normal operations, of a reserve of compressed air or vacuum in trailer emergency air or vacuum tank 82 and also functions to assure that the conventional service brakes are operated simultaneously with and by air pressure or vacuum released by foot pedal operated valve 3 when the trailer is connected to a tractor.

However, in the event that pressure in the tractor air or vacuum tank should fall below a predetermined safe service-brake-operating level, or in the event that the emergency line 19 (or 79') were broken, as, for instance, by accidental or purposeful disconnection of the trailer from the tractor, the emergency relay valve 8| will operate to pass through line 8| to trailer brake operating cylinders 6 the stored compressed air or vacuum from trailer emergency air or vacuum tank 82, thereby immediately setting the brakes. As indicated hereinbefore, the brakes will remain set under air pressure or vacuum alone as long as a sufficient supply of air or vacuum continues to be available in tank 82.

It will be noted that the air or vacuum lines leading to those brake cam shaft locking units associated with the trailer are in open communication with the emergency air or vacuum line 19. Thus, immediately upon failure of this line to carry adequate pressure the diaphragms of the units 25 will collapse causing the internal elements to engage the teeth on the brake cam shaft. The brakes will thus be set by the conventional brake operating means and will be locked in set position until the locking units are deactivated by return of pressure or vacuum to them by reconnection to a properly operative conventional brake system. I

Having described my invention in consideraable detail, I do not wish to be limited to the exact forms shown and described, realizing that modifications thereof will be apparent to those skilled in the art, except as I may be limited by the scope of the appended claim.

I claim:

In a brake system for a tractor and trailer vehicle, a brake-operating system for the tractor comprising individual brake-operating means for singularly operating the brakes of each wheel,

air pressure differential creating an air storage tank on said tractor containing air in stored condition maintained at a differential from atmospheric pressure, conduit means communicating between said tank and each of said brakeoperating means, normally closed valve means in the line of said conduit means operable at will and when open passing to said brake-operating means air from said tank to actuate said brake-operating means and thereby actuate individually but simultaneously the brakes on each Wheel, a brake-locking mechanism associated with each individual brake-operating means, other conduit means in normally open communication between said tank and said brake-locking mechanisms, means in said brake locking mechanisms inhibiting them from acting to lock the brakes during continued maintenance of open communication between said brake-operating means and said tank, and valve means in the line of said other conduit means operable at will for interrupting the maintenance of air at a differential from atmosphe is pressure in said brake-locking means, thereby causing said brakeoperating means to be actuated while said air pressure differential creating means may continue in operation for the continued build-up and maintenance of air at a differential to atmospheric pressure in said tank, and a brakeoperating system for the trailer, including individual brake-operating means for each trailer Wheel, a brake-locking mechanism associated with each of said individual trailer wheel brake operating means, including conduit extension means for said first mentioned tractor conduit means extending beyond said normally closed valve means, extension conduit means on said trailer extending from said tractor air storage tank, a reserve air storage tank on said trailer auxiliary to said tractor air storage tank, and an air control relay valve on said trailer, said tractor conduit means communicating with said air control valve means and therethrough with said brake-operating means of said trailer, said extension conduit means of said trailer communicating with said relay Valve and with each trailer brake-locking mechanism, said reserve air storage tank also communicating with said air control relay valve, and said air control relay valve being adapted under working conditions to maintain a relatively constant supply of air in said reserve air storage tank at a differential to atmospheric pressure and to discharge air therefrom as required for boosting the brake applying force made available through said tractor normally closed valve means and for completely setting the brake-operating means and brakelocking mechanisms should air at a differential to atmospheric pressure be unavailable from said tractor storage tank.

AAGE D. SAHLGAARD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,823,588 Christenson Sept. 15, 1931 2,121,366 Robinson June 21, 1938 2,138,386 Miller Nov. 29, 1938 2,389,507 Eaton July 131, 1945 2,451,139 Williams Oct. 12, 1948 

